Everyone knows the dinosaurs' reign was cut short by an asteroid. But if it hadn't happened, could they have evolved to be even larger?

In 2001, palaeontologists Kristina Curry Rogers and Catherine Forster found a single rib bone in Madagascar that was nearly 3m (9.84 ft) long, roughly the length of a ping pong table. They had discovered a new species of titanosaur – a kind of colossal, plant-eating dinosaur – that was later named Rapetosaurus krausei.

"One of the great things about working with titanosaurs is their 'titanic ' proportions," says Rogers, a DeWitt Wallace professor of biology and geology at Macalester College in Minnesota, USA. "[But] this makes titanosaurs tough to excavate – a single skeleton can take an entire field season or more to extract from the rocks." 

Titanosaurs, literally meaning "gigantic lizards", were the last surviving sauropod dinosaurs, whose members were a diverse bunch of herbivores with very long necks, long tails and relatively small heads. When they were discovered, scientists had already made the famous finds of species like Brachiosaurus, Diplodocus and Apatosaurus, which lived in the Jurassic era, 201 to 145 million years ago. But they thought that sauropods had not survived until the end of the Cretaceous period, which spanned from 145 to 66 million years ago, says Matthew T Carrano, a paleobiologist at the Smithsonian Institution in Washington DC.

Palaeontologists working with titanosaurs often marvel at their sheer scale and size. "A single leg bone is longer than I am, from head to toe," says Rogers, adding that sometimes a single toe bone is as big as an arm bone in a human. "Holding a titanosaur's leg in your hands is pretty remarkable," she says.

But how did they get so large in the first place? And given more time, could they have evolved to be even larger?

All titanosaurs started life relatively small. "They hatched from eggs that were between the size of a softball and a soccer ball," says Rogers. "As brand-new hatchlings they would've been just about the size of a chihuahua. It appears that there wasn't much parental care among these big sauropods, and that babies were out foraging on their own, right out of the nest." Over time, she says, they achieved their colossal size by doing what most sauropods do – eating and eating. 

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Scientists believe that titanosaurs grew quickly and didn't slow down until they reached their massive adult sizes. "When we compare their growth rates throughout life to [those of] living animals, it comes closest to the growth rates we observe in whales. That's incredibly fast," says Rogers.

Titanosaurs, unlike whales, didn't have the benefit of consuming high-protein, nutritious milk provided by their mothers, adds Rogers. Instead, these dinosaurs were out finding their own food. "Keeping their growth rates high is one way in which most sauropods differ from their meat-eating dinosaur relatives, which seem to have paused their growth more often as they got older."

And while titanosaurs were the largest sauropods, Carrano points out that their ancestors among sauropods were already pretty big. "Evolving to be 70-80 tonnes from a 20-30 tonne ancestor probably didn’t require that much evolutionary innovation," he says. He explains that all the structures and systems were already set up for success at huge sizes.

"Sauropods developed long necks early on from their primitive cousins, the sauropodomorphs," says Skye Walker, a field assistant at the Elevation Science Institute for Natural History Exploration, which conducts field work in Montana and Wyoming. "This allowed them to have varied diets, giving them access to a range of nutritious food," she says. 

Because of their increasing size, sauropods developed "pneumatic" air sacs in their bones, to make their skeletons lighter. "These air sacs were made up of soft tissue connected to the lungs," says Walker. "This made their weight easier to bear and allowed for more efficient oxygen supply throughout the body. Unlike mammals, sauropods had this to thank for there being almost no limits to how large they could grow."

Predatory dinosaurs had pneumatic air sacs, as do modern-day birds, says Carrano, and these probably evolved from a common ancestor. However, they are also thought to have evolved independently in other groups, including pterosaurs and sauropods. "This would have increased breathing capacity and lightened these huge bones without sacrificing their strength." Carrano explains that sauropods also had short feet and pillar-like limbs to support their enormous weight. "These are all features taken to extremes in the largest titanosaurs," he says.

Titanosaurs also had adaptations hidden in their joints. Armita Manafzadeh, a postdoctoral fellow studying biology at Yale University, points out that smaller non-avian dinosaurs like T. rex had tight-fitting joints in which their bones interlocked precisely, much like our own. In contrast, titanosaurs had joints with enormous volumes of squishy cartilage at the ends of their bones. This difference in joint structure, she says, especially at key limb joints such as the hip, is thought to be an adaptation for better sustaining the animal’s massive body weight.

However, according to Carrano, what's less clear is how the titanosaurs managed to outgrow their sauropod ancestors. Perhaps they simply had more time to evolve larger bodies, after their Jurassic predecessors were gone. On the other hand, they may have acquired new innovations – tweaking their existing anatomy so that they could get even bigger. "But there’s not a huge difference that makes the answer obvious. It’s also possible that they may have benefited from the availability of new foods, specifically flowering plants, that weren’t around in the Jurassic," says Carrano.  

The jury is still out on whether titanosaurs could have become even bigger, had dinosaurs not gone extinct.

"I think that there could have been somewhat larger sauropods, perhaps, but not dramatically so," says Carrano. He says that it helps to think in terms of "orders of magnitude", which technically refers to change by factors of ten, to think about major shifts in size. For example, going from one tonne to 10 tonnes is a big deal, he says – significantly more so than going from 10 to 20 tonnes. "There’s a shift in scale in the former but not the latter."

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However, there may have been an upper size limit sauropods simply couldn’t have gone beyond, says Carrano. "So we already have 70-80 tonne titanosaurs. Could they have reached 100 tonnes? Perhaps. But 200 tonnes, that I would doubt. Even whales don’t get that big, and they live in a buoyant medium all their lives. I think sauropods were in the maximum size range for terrestrial animals," he says. By the time they went extinct, sauropods had already been around for almost 150 million years. Carrano explains that it's possible the added time wouldn’t have made that much difference.

However, some palaeontologists say further evolution could have been possible, especially since there’s a lot we still don’t know about these creatures.

Titanosaur specimens, though widespread, are largely incomplete, says Walker. "New titanosaurs are still being discovered, so it's really exciting and optimistic to be able to say that we don't know everything yet and we probably never will. We are discovering new things about these animals every single day and bringing more of their mysteries to light."

Over the last 20 years, new titanosaurs have been named at an astonishing rate, says Rogers. "I think that now there are over 100 named species," she says. "It drives home the point that globally distributed dinosaurs like these found a niche and were an incredible evolutionary success story. If not for that rock from outer space, they probably would've continued evolving and diversifying."

BBC

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